The light aromatic hydrocarbons - benzene, toluene, xylene (BTX) - are produced from a number of sources. They are formed in the catalytic reforming of naphtha, a technology primarily directed at the production of high octane gasoline components. The other main source of aromatics is pyrolysis gasoline, or pygas, which is a liquid byproduct formed in the production of olefins by steam cracking liquid feeds, such as naphtha or gas oil. A third source of feedstock is light oil produced during the coking of coal. Although this was once the only source of aromatics, it is now a minor contributor to total supply. It is impossible to separate BTX products of commercial purity by distillation alone. Paraffinic and naphthenic impurities boil in the same range as aromatics and frequently form azeotropes. The production of BTX, therefore, usually employs a solvent with preferential affinity for either aromatics or nonaromatics. Liquid/liquid solvent extraction, extractive distillation, or a combination of the two is used. The market demand for benzene, as a proportion of total BTX, is higher than the proportion of benzene in typical BTX products. Conversion of toluene and, to a lesser extent, xylenes, is practiced by two basic techniques:
- Hydrodealkylation involves stripping the methyl groups from toluene or xylenes to produce benzene and methane.
- Toluene disproportionation where the toluene is converted to benzene and xylenes.
The advantage of disproportionation lies in its high conversion to liquid products and its low consumption of hydrogen compared to dealkylation. It also provides a means of producing xylenes from feedstocks such as pyrolysis gasoline, where the C8 cut is relatively small and has a high proportion of ethylbenzene. One of the disadvantages of toluene disproportionation is that the xylenes product is usually an equilibrium mix of isomers, which does not correspond to the split of market demand. Both UOP and Mobil offer selective Toluene Disproportionation processes, which produce a xylenes stream with around 90 percent of para-xylene, the desired isomer.There exists several nonconventional routes to convert low value refinery byproducts to benzene, toluene, and xylenes. These have been developed and commercialized by various companies over the past several years and include:
- Asahi Chemicals' Alpha Process, which has been operating at the company's Mizushima complex in Okayama, Japan, since 1993, employing an olefinic feed (licensed by Stone & Webster).
- BP/UOP's CyclarTM Process is scheduled to start up its first commercial operation during first quarter 1999 at Saudi Basic Industries Corporation (SABIC).
- Chevron's Aromax Process, which is operating at its Pascagoula refinery
- UOP's RZ PlatformingTM reforming started up its first commercial unit in second quarter 1998.
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